Reduction in both size and power consumption of integrated circuits has led to the proliferation of low power sensors and wireless technology. For example, there are a wide variety of devices using low-power sensors, electronics, and wireless transmitters, separately or in combination, including tablets; smartphones; cell phones; laptop computers; MP3 players; telephony headsets; headphones; routers; gaming controllers; mobile internet adaptors; wireless sensors; tire pressure sensor monitors; wearable sensors that communicate with tablets, PCs, and/or smartphones; devices for monitoring livestock; medical devices; human body monitoring devices; toys; etc. These devices may include various microelectromechanical devices to provide a number of different functions. For example, the microelectromechanical devices may include various sensors to monitor and detect environmental conditions such as pressure, vibration, flow rate, strain, physical motion relative to a force (i.e., gravity), impulse motion, or sound. Examples of such sensors include accelerometers, gyroscopes, pressure sensors, strain sensors, flow sensors, and microphones. These devices may further include actuators which provide energy to mechanically drive a system within the device. Additionally, the devices may include energy harvesters that essentially convert movement (e.g., vibrational energy) into electrical energy. Design and manufacturing processes for the microelectromechanical devices vary depending on the application. Further, the various microelectromechanical devices may be distributed through the device in an inefficient manner.
Other wireless technologies of significant interest are wireless sensors and wireless sensor networks. In such networks, wireless sensors are distributed throughout a particular environment to form an ad hoc network that relays measurement data to a central hub. Particular environments include, for example, an automobile, an aircraft, a factory, or a building. A wireless sensor network may include several to tens of thousands of wireless sensor “nodes” that operate using multi-hop transmissions over distances. Each wireless node will generally include a sensor, wireless electronics, and a power source. These wireless sensor networks can be used to create an intelligent environment responding to environmental conditions.
Microelectromechanical (“MEMS”) piezoelectric devices with silicon structural layers typically have a cross-section consisting at least of oxide/structural layer/piezoelectric stack/oxide (the oxide is typically deposited silicon dioxide). The silicon material used for the structural layer is typically formed from the single crystalline silicon device layer of a silicon-on-insulator (“SOI”) wafer. A second piezoelectric stack is often placed in the device structure to form a dual piezoelectric stack in order to increase the signal output from the device. The additional piezoelectric stack is placed on the same side of the silicon structural layer as the first piezoelectric stack (or on top of the first piezoelectric stack), in the format of at least oxide/structural layer/piezoelectric stack/oxide/piezoelectric stack/oxide. The additional piezoelectric stack is placed in this manner because during the process used to fabricate SOI wafers, direct bonding of two silicon wafers at high temperature, followed by grinding and polishing steps, makes it extremely difficult to place piezoelectric stacks in between the device and handle wafers.
Placement of the additional piezoelectric stack on the same side of the silicon structural layer as the first piezoelectric stack creates a lack of symmetry in cross-section with respect to the piezoelectric stack, requiring the piezoelectric stack residual stress to be tuned in order to engineer the flatness of the structural layer. Curl or lack of flatness in the structural layer due to poor management of residual stress in the layers can impact the performance of the MEMS device. Tuning the residual stress of the piezoelectric stack can also impact its inherent piezoelectric properties and, thus, device performance. A tradeoff must, therefore, be made in the stresses required for structural layer flatness and for quality piezoelectric response.
The present invention is directed to overcoming these and other deficiencies in the art.